The signal-to-noise ratio (SNR) and/or the carrier-to-noise ratio (CNR) in a cable television transmission picture are among the most important parameters characterising the performance of a cable network and the quality of the service being provided. The higher the signal-to-noise ratio obviously the better the quality of the picture that can be obtained. The Television Allocation Study Organization (TASO) have set a minimum for a grade 1 picture quality of at least 41 decibels (dB) and with the larger screen sizes now available it is likely that this ratio will be increased even further to maintain the quality of picture.
Picture tubes currently in use in the majority of television sets begin to shown poorer quality picture when the signal-to-noise ratio reaches about 40 (dB). It will be apparent that an effective system for monitoring the signal-to-noise ratio is important and will become even more important in the future to ensure transmission of better quality cable TV signals.
The signal-to-noise ratio of a cable network is currently measured by sending a test signal from the head end. This signal must be inserted during the vertical field retrace time and can only occupy a small portion of a frame. An acceptable measurement requires the averaging of the results obtained from processing of hundreds of consecutive frames of test signals as otherwise the results are unreliable.
In some cases the TV images may be impaired by snow noise before activation of a test signal in this case a high signal-to-noise ratio of a test signal still does not guarantee a good picture at the subscriber location as the transmitted signal already contained noise before activating the test signal at the head end,
A portion of the bandwidth for each channel is used for band (channel) separation and does not carry a signal, rather the luminance signal (black and white portion of the signal) and the chrominance signal (coloured portion of the signal) are carried within the first ninety six percent of the band-width allocated to a particular channel leaving the remaining portion of band-width carrying no picture information. This limited portion of the band width for a given channel may be used to determine the signal-to-noise ratio without interfering significantly with the picture information. However, testing for signal-to-noise ratio in this unused portion of the band does not produce an accurate assessment of the signal-to-noise ratio of the actual interference in the remaining band portion carrying the information to produce the picture at the subscriber end.
In an article entitled "Measurement of Signal to Noise Ratio of a Television Broadcast Picture" by CD Janse Van Rensburg DeJageor, IEEE Transactions on Broadcasting, volume 37, No 2 pages 35 to 43, 1991 a method of using the high frequency content of a TV image to evaluate signal-to-noise ratio has been described. This method is limited to cartoon pictures or near constant luminance TV signals.